1. Field of the Invention
The present invention relates generally to acoustic sensors and more specifically to acousto-optic sensors.
2. Description of the Prior Art
Acousto-optic interferometric sensors are based upon the phase shift in light passing through a fiber which occurs when that fiber is exposed to acoustic waves. Such sensors generally include acoustically insensitive lead fibers attached to a sensor section which includes an acoustically sensitive sensor fiber and an acoustically insensitive reference fiber. A laser beam is split and transmitted through both the reference and sensor fibers. The difference in phase shift between the reference and sensor fibers is related to the strength of the sensed acoustic field. These types of sensors are described in further detail in various patents, such as U.S. Pat. Nos. 4,162,397, 4,297,887 and 4,363,114, all of which are incorporated herein.
Because the phase shift which occurs within any one length of fiber immersed in liquid is small, the fibers are arranged in a multipath configuration, such as a coil or spiral. Acoustic sensitivity is maximized when the sensor fiber is configured as a coil wound about a polymer mandrel such as Teflon or nylon. In such an arrangement, the transduction mechanism is indirect. The acoustic field generates strains within the mandrel which changes its diameter and thus the fiber length, which, in turn, modulates the phase. Unfortunately, that arrangement is unsuitable for a planar sensor.
Piezoactive polymers, such as polyvinylidene fluoride (PVF.sub.2) have been successfully utilized as the transduction mechanism for acoustic sensing. Piezoactive polymers provide a unique shape flexibility which permits their fabrication into two-dimensional sensors. However, the small electric output of the piezoactive polymers must be amplified at the wet end and is subject to electromagnetic interference.